Does the term "selling price" mean the "cost price" or the "sale price" of a product/commodity? I have been told that the idiom "selling price" is the same as the cost price of an item, that is the amount which a seller pays to, e.g. a wholesale merchant. The seller later sells the commodity at a higher sale price, and thus earns a profit.
But then the following definition:

the price at which something is offered for sale

--Here's the source.
Then an example is this question in a textbook:

Sue bought a TV set for $15000 on installments at the markup rate of
  12% per annum. Find the selling price of the TV if the time period
  is 3 years.

The definition and the example question statement show that the selling price is the sale-price, and NOT the cost-price.
But I had been stuck in a question earlier which I posted here (see accepted answer and comments under it), and I found out that my mistake was that I was calculating the GST (General Sales Tax) based on the Sale-price. I was doing that on the basis of the following definition, 

General Sales Tax is imposed by the Government on the percentage of
  the selling prices of things.

I learnt from all that that the Selling Price is the same as the Cost Price and not the Sale Price.
Now what's incorrect and what's correct?
 A: I think the only answer to your question is "it depends". In my opinion (as in my answer to the question you link to) is that "selling price" usually means "price before tax". 
If a problem you encounter in a text or on an exam is ambiguous the best thing to do is say "If it means this the answer is that, else the answer is something else" thus proving that you can do the arithmetic either way and can read.
The question about the TV is ambiguous (as well as unrealistic - $\$15,000$  for a TV?). In this case I suspect the $\$15,000$  is the ticket price. To find out how much she actually pays is really tricky - like paying off a mortgage. How often does she make payments? The reverse question seems quite unrealistic to me. Only in a math class would you have to figure out what the ticket price was on the television set if her total payments were $\$15,000$.

A general strategy for many percentage problems is to use "1+rate". For example, to compute a 12% increase in 123, calculate $1.12 \times 123$ rather than finding 12% and then adding it on. For three 12% increases in a row, multiply by $1.12^3$ rather than doing them one at a time (this is compound interest). To find the value before a 12% increase (your price before tax problem) you divide by 1.12. To calculate a 12% discount, multiply by $1-0.12 = 0.88$.
Read more if you wish in Chapter 3 at http://www.cs.umb.edu/~eb/qrbook/qrbook.pdf .
A: Let me try to make some things clear-
The guy, the retailer, gets the product from a dealer for say $80$. He wants a profit of say $20$ making it $100$ . Now, He has to pay tax. He doesn't want to pay tax himself, but will try to get the customer to pay the tax. He adds say $20 \%$ tax and finally, a customer buys it for $120$.
Cost Price - $80$
Selling price - $100$
Selling price(inclusive of all taxes) - $120$
There are two selling prices - One in terms of the retailer and one in terms of the customer. The retailer will give the $20$ to the tax and finally, he thinks the selling price is just $100$ . Customer though got it for $120$. This is his selling price. 
Basically, it depends on the problem, as to - from whose side is it asking. I hope I am clear.
